The detection of refractive index changes of dielectric media adjacent to a metallic surface by means of detection of the Surface Plasmon Resonance (SPR) is well known.
A surface plasmon wave is a magnetic transversal electromagnetic wave which propagates at the interface of a metal and a dielectric, where the metal behaves similarly to a free-electron gas. The plasma wave is characterized by a propagation vector (wave vector), which defines the conditions required for it to be excited. If the metallic and dielectric mediums are semi-infinite, the plasmon propagation vector kSP is given by the following equation:
      k    SP    =                              2          ⁢          π                λ            ⁢                                    n            m                    ⁢                      n            d                                                              n              m              2                        +                          n              d              2                                            =                            2          ⁢          π                λ            ⁢                                                  ɛ              m                        ⁢                          ɛ              d                                                          ɛ              d                        +                          ɛ              d                                          where λ is the wavelength, and nm and nd are, respectively, the refractive indexes of the metal and the dielectric (and ∈m and ∈d are their dielectric constants, with n=√{square root over (∈)}).
For the plasmon resonance phenomenon to be produced, the real part of the dielectric constant of the metal must be negative, Re[∈m]<0, Re[∈d]<−Re[∈m], and the wave produced must be transversal magnetic (TM). These conditions are fulfilled for several metals, amongst which the most widely used are gold and silver. The electromagnetic field of a surface plasma wave is characterized in that it has maximum intensity in the interface of the metal and the dielectric, and exponential decay in both media, as briefly shown in FIG. 1 (this figure shows the wave exponential decay in the interface of the metal 100 and the dielectric medium 200)
As a consequence, the excitation of the surface plasma wave will strongly depend on the dielectric constant (or refractive index) of the dielectric medium.
There are several ways of exciting these surface waves, e.g. by means of electrons or by means of light. However, the excitation of this surface plasmon wave cannot be produced by directly reflecting light on the metal. The reason for this being that the light wave vector follows the equation:
      k    LIGHT    =                    2        ⁢        π            λ        ⁢                  ɛ        d              ⁢    sin    ⁢                  ⁢    θ  
where θ is the angle of light incidence and λ is the wavelength. In order for the excitation to be produced, both wave vectors must be equal. Comparing the wave vectors of the plasmon and the wave vectors of the light, it follows that, for any angle of incidence of light:|kLUZ|<|kSP|
Several techniques are used to excite surface plasmon with light, amongst which we can highlight the following:
a) Prism coupling (diagrammatically shown in FIG. 2): a prism 10 is used, with a refractive index np and a dielectric constant ∈p larger that those of the dielectric medium 200 wherein the optic changes are to be produced (∈p>∈d), as well as a thin metal plate or layer with an specific thickness (depending on the light wavelength and on the metal used) placed between the prism 10 and the dielectric medium 200. In FIG. 2, kx0 is the component of the light wave vector in the air, parallel to the reflection surface (and ∈0 is the dielectric constant of the air), kxp is the component of the light wave vector in the prism, parallel to the reflection surface (and ∈p is the dielectric constant of the prism), and kSP is the propagation vector of the plasmon.
Excitation is produced by the complete internal reflection of the light on the interface between the prism and the metal, and plasmon is generated on the interface of the metal and the dielectric medium, whereon the measurement is to be made. In this configuration, the thickness of the metal layer is an essential parameter in order to observe the plasmon resonance. The optimal thickness can be estimated by several methods, e.g. through the formalism described in the article by M. Shubert, Polarization-dependent optical parameters of arbitrarily anisotropic homogeneous layered media, Physical Review B, vol. 53, p. 4265 (1996).
b) Designing a periodic structure, such as a grid, on the metal layer. In this way, a light diffraction phenomenon occurs, falling on the periodic structure and leading to an increase in the light wave vector:
      k    LIGHT    =                              2          ⁢          π                λ            ⁢                        ɛ          d                    ⁢      sin      ⁢                          ⁢      θ        +          N      ⁢                          ⁢                        2          ⁢          π                Λ            
where Λ is the period of the periodic structure and N is the order of light diffraction. The thickness of the metallic layer is not a very important parameter with this method, however, the period and depth of the periodic structure will be important.
c) By guiding light on a wave guide or on an optical fiber. Excitation is produced through the evanescent field of the light confined within the core of the guide or optical fiber.
These forms of exciting surface plasmon by light incidence are conventionally used (perhaps, particularly in the prism coupling-based system) in systems to measure/detect changes in the refractive indexes of dielectric media.
These measurement and detection systems are based on the fact that the excitation condition of the plasmon resonance depends on the refractive index, nd of the dielectric medium. This means that if the refractive index changes, the excitation condition of the plasmon will also change. This change in the resonance condition can be detected in different ways, e.g. by analysing the light reflected by the metal layer as a function of the angle of light incidence, by keeping the wavelength fixed, and in a configuration of prism coupling.
FIG. 3A shows a known configuration to detect changes in the refractive index of a dielectric medium, comprising a monochromatic light source 20 with transversal magnetic polarization (also known as “TM polarization” or “p polarization”, i.e. with the electric field within the light incidence plane), a light intensity detector 30 connected to electronic data processing means 40 set up to analyse signals at the light intensity detector 30 output. In addition, the configuration comprises a coupling prism 10, a thin metallic layer 100 (normally gold) positioned on a prism surface 10, and, the dielectric media 200 (e.g. a fluid) on the other side of the metallic layer, i.e. in contact with the metallic layer surface, which is not in contact with the prism 10. The light 21 is reflected when falling on the metallic layer and the reflected light falls on the light detector 30, which detects its intensity, in turn recorded by the electronic data processing means 40.
FIG. 3B diagrammatically shows how the prism 10 and the metallic layer 100 can be rotated in relation to the light source 20, so that the angle of light incidence θ 21 varies (this can be done by moving the light source and/or the unit formed by prism 10 and metallic layer 100).
As it gathered from the above explanation, the condition of plasmon excitation with light depends on several factors, such as the wavelength of the light, the angle of incidence θ, and the refractive index nd. If the shown configuration starts from a small angle of incidence and this angle is increased, it reaches a point where the total reflection of the light on the prism 10 and the metal plate or layer 100 interface occurs. From this angle, if the angle of incidence θ continues to be increased, a strong decrease occurs in the reflected intensity, up to a minimum, which coincides with the excitation of the surface plasma wave on the other metal interface. Given that the excitation condition of the plasmon resonance depends on both the angle of incidence θ and the refractive index (nd) of the dielectric medium, if the other variables are kept constant (e.g. the dielectric constant ∈m and other features of the metallic layer, the light wavelength, etc.), a change in the refractive index (nd) of the dielectric medium will correspond to a change in the angle of incidence θ, for which a minimum in the intensity of the reflected light occurs.
FIG. 4 shows two curves which relate the intensity Rpp of the reflected light to the TM polarization (measured with the detector 30 of said configuration) as a function of the angle of incidence θ, for two different refractive indexes (nd1,nd2 with nd1<nd2). As can be observed, the increase in the refractive index from nd1 to nd2 is shown with a certain shift to the right in the Rpp(θ) curve diagram, due to the increase in the angle of incidence for which the plasmon excitation occurs. In this way, by making θ sweeps, the change in the angle for which the plasmon excitation occurs can be detected, and such change can be related to the variations in the refractive index of the dielectric medium 200.
That is to say, the quantification of the angle shift for which the resonance occurs, provides a measurement of the refractive index change. On the other hand, the sensitivity with which these changes of resonance angle can be detected, depends on how narrow the resonance curve is. The narrower the curve, the higher the sensitivity, and that will depend, in this case, on the metal used, on the layer thickness, and on the light wavelength. The configuration normally used is a 50 nm gold layer and light with a 632 nm wavelength.
An alternative way to detect changes in the refractive index can be to keep the angle of incidence θ constant, and to measure the changes in reflectivity (in the case of FIG. 4, if it is decided to keep the angle of incidence θ=72 degrees, an increase in the refractive index from nd1 to nd2 will be detected as a reflectivity increase, etc.). As in the previous case, the sensor's sensitivity depends on how narrow the resonance peak is.
If instead of varying the angle of incidence θ, we change the light wavelength, exactly the same occurs, the appearance of a resonance peak which moves when the refractive index of the dielectric medium adjacent to the gold layer is changed. This also applies to the case of excitation by means of a periodic structure or by means of a wave guide.
There are a great number of systems for detecting changes in refractive indexes based on surface plasmon resonance; examples of said systems are disclosed in:
U.S. Pat. No. 5,912,456
U.S. Pat. No. 5,485,277
U.S. Pat. No. 2003103208
Naturally, a direct application of this kind of sensor is the refractometer (for measuring refractive index changes). However, another important application of this kind of sensor at present is the biosensor or chemical sensor. The penetration distance of the evanescent field of the surface plasma wave within the dielectric medium is around 100 nm. Therefore, a biomolecular interaction occurring on the metallic layer surface will locally modify the surface refractive index. This variation will produce, in turn, a change in the propagation vector of the plasmon, and, as a consequence, in the resonance condition. This change can be detected by the abovementioned methods.
The use as biosensor can be based on the prior immobilization of receptor biomolecules 210 on the metallic layer surface 100, as diagrammatically shown in FIG. 5. These receptor biomolecules can be selectively bound to analyte molecules 220 which are to be detected and which can be present in a liquid that the metallic layer is in contact with. When the analyte molecules 220 are bound to the receptor molecules 210, a local change in the refractive index on the metallic surface will again occur which, in turn, will change the plasmon resonance condition.
Currently, there are multiple commercial devices and a great number of articles describing different types of measurement configuration and applications of this kind of sensors.
Surface plasmon resonance sensors generally are highly sensitive to detecting changes in refractive index, as well as low biomolecules concentrations. However, they may have sometimes insufficient sensitivity, e.g. currently, known sensors have problems in detecting changes in refractive index under 10−5 and molecules with a low molecular weight (lower than 1000 units of atomic mass), when used as biosensors. This means that detection of certain substances, such as chemical toxic substances or environmental polluting agents, is complex and cannot be performed directly and appropriately (using the abovementioned technology).